P-cadherin is a classical cadherin expressed by myoepithelial cells in mammary tissue. Its expression in human breast cancer has been associated with aggressive tumour behaviour. To analyse the possible role of P-cadherin in canine mammary carcinogenesis, its expression was examined immunohistochemically in 82 samples of normal (n=2), hyperplastic (n=11) and neoplastic (n=69) canine mammary tissues. In normal and hyperplastic canine mammary glands, P-cadherin was restricted to myoepithelial cells, usually at sites of cell-to-cell contact. In tumour tissues, however, P-cadherin expression was observed in both epithelial and myoepithelial cells, with a cytoplasmic pattern of cellular distribution. Aberrant epithelial P-cadherin immunolabelling was found in 19/44 (43%) benign tumours and in 16/25 (64%) malignant tumours (P<0.001). In malignant tumours, a significant correlation between P-cadherin expression intensity and histological type was observed (P<0.05).